1. Field of the Invention
The present invention relates to the fields of molecular biology, cell signaling and cancer biology. More specifically, the present invention demonstrates the use of TSG101 as biomarker for the diagnosis and prognosis of cancer, especially ovarian cancer.
2. Description of the Related Art
Oncogenic transformation is an intricate process involving alterations of multiple genetic elements and signaling cascades. One critical signaling molecule that contributes directly to transformation is the small G-protein RAS. RAS functions as an intracellular molecular switch, cycling between the GDP-bound inactive state and the GTP-bound active state in response to external stimuli leading from cell surface receptor tyrosine kinases to nuclear transcription factors (Berchuck and Carney, 1997; Marshall, 1995). Several well-known intracellular signaling cascades including the Raf/MEK/ERK pathway, the PI3 kinase pathway, and the Ral-GDS pathway, have been identified as mediators of RAS downstream effects (Feig et al., 1996; Kauffmann-Zeh et al., 1997; Khosravi-far et al., 1996; Shibatohge et al., 1998; Vavvas et al., 1998; Watari et al., 1998). RAS-associated cell signaling is involved in many important cellular processes, such as cell growth, differentiation and survival under physiological conditions. Constitutively active RAS mutants have been found in 30% of all human cancers. RAS or BRAF mutations are involved in 60-70% of low grade serous ovarian cancer (Shih and Kurman., 2004). However, it is still unclear as to how oncogenic RAS mutants, in collaboration with other oncogenes and tumor suppressors, perturb the balance of cellular signaling networks and lead to the formation of cancer cells.
One particular protein implicated in tumorigenic processes that has garnered significant interest in recent years is the tumor susceptibility gene 101 (TSG101), a coiled-coil domain-containing protein that interacts with stathmin in a yeast two-hybrid screen. This gene encodes a multidomain protein that contains a putative DNA-binding motif at its C-terminus and can act as a transcriptional cofactor to repress or activate nuclear hormone receptor-mediated transactivation. The N-terminal region of TSG101 shares an extensive sequence homology to the Ubc domain of ubiquitin-conjugating enzyme (E2) but lacks a critical active-site cysteine residue essential for enzymatic activity, thus suggesting a potential role for TSG101 in regulation of ubiquitin-mediated protein degradation. Several studies have shown TSG101 to be an important cellular factor that specifically recognizes mono-ubiquitinylated proteins and mediates endosomal trafficking that is important for membrane receptor endocytosis and retroviral budding.
The implied tumor suppressor function of TSG101 was proposed based on a homozygous functional knockout study, in which inactivation of TSG101 in NIH3T3 mouse fibroblasts led to focus formation in monolayer cell cultures, anchorage independent growth in soft-agar, and in vivo tumor formation in nude mice (Li and Cohen., 1996). Initial studies suggested that TSG101 was often mutated in human breast cancers (Li et al., 1997) and its aberrant splice variants were frequently detected in different tumor types (Gayther et al., 1997; Lee and Feinberg, 1997; Li et al., 1998; Steiner et al., 1997; Sun et al., 1997; Wang et al., 1998). However, it was determined later that these apparent mutations were in fact alternative splice variants generated exclusively by exon skipping (Wagner et al., 1998). The functional roles of these TSG101 splice variants in normal cellular processes and tumorigenesis remains unsolved.
Although TSG101 is essential for cell proliferation, cell survival and embryonic development under normal physiological conditions (Carstens et al., 2004; Krempler et al., 2002; Ruland et al., 2001; Wagner et al., 2003), the role of TSG101 in tumor formation and development has proven to be complex and remains controversial. For instance, TSG101 was initially discovered as a potential tumor suppressor and the expression of TSG101 has been shown to be decreased in certain cancer samples (Bennett et al., 2001). However, more recent studies suggest that TSG101 levels are elevated in some human cancers, including thyroid (Liu et al., 2002) and gastrointestinal tumors (Koon et al., 2004). Functional proteomic approaches, have recently revealed that TSG101 is overexpressed in a large number of ovarian cancer patients. Although TSG101 was initially recognized as a potential tumor suppressor (Li and Cohen, 1996), the precise role of TSG101 in tumor formation, development, and its relevance to ovarian carcinomas in the clinical settings are largely unknown. Furthermore, overexpression of TSG101 can also lead to neoplastic transformation (Li and Cohen, 1996). Gene silencing of TSG101 leads to growth arrest and cell death in breast and prostate cancer cells (Zhu et al., 2004), instead of growth promotion as would be expected for loss of a true tumor suppressor.
Although it is known that steady-state TSG101 levels are tightly controlled in normal cells, primarily at the post-translational level, keeping protein concentrations within a narrow range (Feng et al., 2000), the mechanism of TSG101 post-translational regulation is not clear. Hence, understanding the cellular regulation of TSG101 is an important task for further elucidating the function of TSG101 under physiological and neoplastic conditions.
Thus, the prior art lacks an understanding of the precise role of TSG101 in human cancer development, its correlation with clinicopathological variables, survival, tumor formation and development specifically in ovarian cancer in clinical settings. The present invention fulfills this long-standing need and desire in the art.